Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well known advantages over other flat-panel display devices currently in the market place. Among these advantages is brightness of light emission, relatively wide viewing angle, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs).
Applications of OLED devices include active matrix image displays, passive matrix image displays, and area lighting such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is the lower or bottom electrode of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is the upper or top electrode, the device is referred to as a top-emitting OLED device.
The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
It is also well known that OLEDs are sensitive to moisture. If water molecules reach or penetrate an OLED device, the operational lifetime of such device can be reduced significantly.
One approach to increase the operational lifetime of an OLED device to acceptable levels is to reduce the adverse effects of moisture by positioning the device in a separate enclosure or package together with moisture absorbing desiccants, and to seal the enclosure so that moisture penetration into the enclosure is reduced or eliminated. Unfortunately, the cost of such moisture protective packaging is likely to constitute a substantial proportion of the total device cost, particularly as device size or device area increases.
Another approach to providing moisture protection of OLED devices has been proposed by Ghosh et al. in International Patent Application WO 01/82390 A1. In this disclosure, a plurality of spaced apart OLED devices are formed on a rigid substrate which is presumed to be impervious to moisture penetration.
An encapsulation assembly is disposed over the OLED devices and over at least a portion of the substrate. The encapsulation assembly includes at least two layers. A first layer can be a dielectric oxide layer, which is directly in contact with at least a portion of the rigid substrate, and a second layer is a polymer layer, which covers the first layer. Alternatively, it is proposed to provide a first polymer layer over the OLED devices and patterned to be in direct contact with the substrate around a device perimeter. A second layer of the proposed encapsulation assembly is a dielectric oxide layer in this latter configuration. Thus, Ghosh et al. provide moisture protection which is integral with an OLED device and, accordingly, does not require device sealing within or by a separate enclosure. It will be appreciated that the Ghosh et al. encapsulation assembly is disposed over OLED devices that are formed directly on a rigid substrate such as, for example, a glass plate or a silicon wafer.
It is anticipated that OLED devices having relatively large dimensions will be manufactured on a flexible substrate, also referred to as a web, in the form of passive matrix display panels for relatively large and low cost displays, or in the form of relatively large area illumination sources for selected lighting applications such as, for example, selective desk top lighting.
It is further anticipated that flexible substrates will include plastic-foil materials selected to provide adequate physical properties for the intended OLED applications, and at the lowest possible cost of such materials. Relatively low cost plastic flexible substrates, for example acrylic material foils, are known to be ineffective moisture barriers. Accordingly, OLED devices formed directly over such flexible substrates would be subject to moisture penetration and attendant reduction in operational lifetime, irrespective of moisture protection provided integrally over the OLED devices.